Project Summary Selective protein trafficking in the secretory pathway is vital for cell function and growth. Our research program is focused on coat-dependent sorting mechanisms that catalyze transport between the endoplasmic reticulum (ER) and Golgi complex. Here nascent secretory proteins are translated at the ER and then fully folded proteins are selectively packaged into COPII coated vesicles for anterograde transport to the Golgi complex. This forward pathway is balanced by retrograde transport from the Golgi, which selectively returns proteins to the ER in COPI coated vesicles. To ensure delivery of only folded secretory proteins, a process known as ER quality control retains nascent proteins in the ER until correctly folded or ultimately targets terminally misfolded proteins for degradation. Mechanisms that coordinate efficient sorting of cargo into transport vesicles with ER quality control are not well understood. We have identified a set of transmembrane cargo receptors that perform important functions in coat-dependent sorting and quality control in the early secretory pathway. This research proposal will address key questions in the field regarding how cargo binding to receptors is controlled to achieve net directional transport of proteins and how cargo receptors function in ER quality control. The specific aims of the proposal are to: (1) determine how the Erv41-Erv46 cargo receptor catalyzes retrieval of escaped ER resident proteins; (2) define molecular sorting signals in cargo that are required for Erv41-Erv46 recognition; (3) investigate how Erv41- Erv46 retrieves misfolded proteins from post-ER compartments; and (4) determine how the Erv26 and Erv29 anterograde cargo receptors function in ER quality control. We will rigorously test our hypotheses in a yeast model by exploiting molecular genetics to monitor protein function in vivo, through cell free assays with isolated ER and Golgi membranes in vitro and in reconstitution experiments with purified factors. Defining the molecular mechanisms that underlie these fundamental cellular processes should improve current approaches to treat human diseases connected to secretory pathway function.